Electronic components installed in devices, e.g., portable phones and the like have been made compact these days in association with a great miniaturization of the devices. There is a bump forming apparatus which forms bumps on electrode parts in circuit form parts formed on a semiconductor wafer without separating the circuit form parts individually from the semiconductor wafer. The bump forming apparatus of this kind comprises a carry-in device for taking out the semiconductor wafer without bumps formed yet, namely, a pre-forming bumps wafer from a first storage container where the wafers are stored before forming bumps, a second storage container for storing semiconductor wafers with formed bumps (i.e., a wafer with formed bumps) a bonding stage whereon the wafer is placed before forming bumps and which normally heats the semiconductor wafer to 250-270° C. so as to join the electrode parts and bumps, a carry-out device for moving the wafer with formed bumps into the second storage container, and a transfer device for transferring the wafer from the carry-in device to the bonding stage and from the bonding stage to the carry-out device.
Meanwhile, there are piezoelectric substrates on which SAW (Surface Acoustic Wave) filters, used in the portable phones or the like, are formed, substrates consisting of quartz unlike conventional ones of silicon, and compound semiconductor wafers having substrates formed of lithium tantalum, lithium niobium, gallium arsenide and so on. Although the compound semiconductor wafers or the like are normally heated to about 150° C. to a maximum of approximately 200° C. when bumps are formed, a speed in heating and cooling the wafers must be reduced in comparison with the conventional silicon wafers.
For example, a SAW filter 10 shown in FIG. 85 has an input side circuit 12 and an output side circuit 13 formed as a pair on a piezoelectric substrate 11. Bumps 19 are formed on electrode parts 18 of the SAW filter 10 by a bump forming head of the bump forming apparatus as shown in FIG. 88. Both of the input side circuit 12 and output side circuit 13 have a shape like a fine-toothed comb. The input side circuit 12 is oscillated by a supplied input electric signal. The oscillation propagates a surface 11a of the piezoelectric substrate 11, thereby vibrating the output side circuit 13. An electronic signal is generated and outputted by the output side circuit 13 based on the vibration. The SAW filter 10 thus passes only signals of a specific frequency. The SAW filter 10 shown in FIG. 85 is one of many SAW filters 10 formed in a matrix on the wafer-shaped piezoelectric substrate 11. Operations, for instance, forming bumps, etc. on circuit parts of the SAW filters 10 are carried out on the wafer-shaped piezoelectric substrate 11. Each SAW filter 10 is separated from the wafer-shaped piezoelectric substrate 11 at a final stage. The wafer-shaped piezoelectric substrate 11 has a characteristic that the substrate 11 is hard to charge, but it is difficult to remove electric charge from the substrate once it is charged.
Because of using the piezoelectric substrate 11 as above, electric charge is generated by deformation or the like of the wafer-shaped piezoelectric substrate 11 caused by a temperature rise and a temperature drop between a room temperature and the aforementioned approximately 150° C., so that front and rear faces of the wafer-shaped piezoelectric substrate 11 become charged. A quantity of the charge becomes approximately 9000V at maximum.
Since the wafer-shaped piezoelectric substrate 11 is thin by itself, the rear face thereof is possibly induced to vibrate by the vibration generated at the front face 11a, which adversely affects the vibration of the front face. For preventing generation of the vibration on the rear face, fine grooves 14 are formed as indicated in FIG. 87 on the rear face of the wafer-shaped piezoelectric substrate 11. Electric charge present inside the grooves 14 is difficult to remove. Although the grooves 14 are exaggeratively illustrated in FIG. 87, the grooves 14 are actually formed in a size conforming to a frequency to be processed by the SAW filter, and arranged by a pitch of approximately several μm-several hundreds Å(angstrom).
If the charged wafer-shaped piezoelectric substrate 11 is placed on, for example, the bonding stage, sparking takes place in some cases between the bonding stage and the piezoelectric substrate 11 or between the front and rear faces of the wafer-shaped piezoelectric substrate 11. If the sparking occurs, the sparking melts the comb teeth part thereby breaking the circuit as indicated by reference numerals 15-17 in FIG. 86. Also, when the wafer-shaped piezoelectric substrate 11 is brought to, for example, above the bonding stage, the wafer-shaped piezoelectric substrate 11 is attracted towards the bonding stage by charge, and consequently the wafer-shaped piezoelectric substrate 11 may be broken by the attraction force. Furthermore, when the piezoelectric substrate 11 is to be moved again after putting on the bonding stage, the piezoelectric substrate may be broken if the substrate is forced to move because a uniting force to the bonding stage is so strong.
As above, in the bump forming apparatus for forming bumps onto the substrate which generates electric charge consequent to a temperature change in the temperature rise and temperature drop such as the wafer-shaped piezoelectric substrate 11, quartz substrate wafer, compound semiconductor wafer and the like, it becomes an important issue to eliminate charge, whereas it was not a fundamental problem in the conventional bump forming apparatus for forming bumps on silicon wafers.
In the meantime, as is disclosed, for example, in the published specification of Japanese Patent Laid-Open Publication No. 55-87434, a wafer is proposed in which an aluminum film is formed along a dicing line provided on a front face of the wafer to let electric charge of the front face out to a periphery of the wafer along the dicing line, thereby removing the charge through the periphery, or in which an aluminum film is formed on the entire rear face of the wafer to facilitate elimination of charge from the rear face. It is possible to remove charge from the wafer in this manner. But the aluminum film of the rear face may be separated to cause troubles at the application of a pressure and ultrasonic vibration to the rear face by a pressing member because the pressure and ultrasonic vibration are applied with the pressing member brought into contact with the rear face, for example, when each chip cut out from the wafer is flip chip mounted to the substrate via the bump. Therefore, the aluminum film formed for the purpose of elimination of charge should be removed before the chip is mounted, resulting in an increase of processes and costs.
On the other hand, since charge is generated consequent to the temperature change in the temperature rise and temperature drop of the wafer-shaped piezoelectric substrate 11, quartz substrate wafer or compound semiconductor wafer as discussed above, a speed of the temperature rise and temperature drop should be set lower than in conventional silicon wafers. As a result, a cycle time becomes undesirably lengthy in the case of the piezoelectric substrate 11, as compared with conventional silicon wafers not accompanied with generation of charge. Moreover, for example, when the temperature change takes place as the wafer-shaped piezoelectric substrate 11, quartz substrate wafer or compound semiconductor wafer is placed on the bonding stage after being raised in temperature, the wafer-shaped piezoelectric substrate 11 is warped due to a difference between a raised temperature and a temperature of the bonding stage. This warpage should be corrected, because the wafer-shaped piezoelectric substrate 11 would crack, be chipped or break if bumps were formed on the warped substrate.
The present invention is devised to solve the above-described problems, and has for its object to provide a bump forming apparatus which can effectively remove charge generated as a result of a temperature rise and a temperature drop of charge appearance semiconductor substrates before and after bumps are formed on the substrates, can operate with a cycle time not inferior to a cycle time for substrates not accompanied with generation of charge even in the presence of a temperature difference, and will not break the charge appearance semiconductor substrates (that is, can prevent the charge appearance semiconductor substrates from pyroelectric breakdown and physical failures. A further object is to provide a method carried out by the bump forming apparatus for removing the charge of charge appearance semiconductor substrates, to provide a charge removing unit installed in the bump forming apparatus for charge appearance semiconductor substrates, and to provide a charge appearance semiconductor substrate.